JP2007227212A - Fuel cell scavenging method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fuel cell scavenging method capable of efficiently and quickly removing moisture in the fuel cell. <P>SOLUTION: When power generation stops in a fuel cell 1, this method circulates a scavenging gas through a fuel gas channel 5 and an oxidant gas channel 6 in the fuel cell 1 to scavenge both gas channels 5, 6 of the fuel cell 1. This method circulates a scavenging gas having a pressure higher than that of supply gas prior to power generation stop in the fuel cell 1 through either the fuel gas channel 5 or oxidant gas channel 6, performs a first scavenging in the target gas channel and then circulates the scavenging gas through the other gas channel to perform a second scavenging in the other gas channel. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a scavenging method for a fuel cell that generates power by supplying a fuel gas and an oxidant gas.

In general, in a fuel cell, in order to prevent water from freezing in the fuel cell during a power generation stop period in a low temperature environment and reducing the power generation performance during the next power generation, A scavenging gas is circulated through the gas flow passage and the oxidant gas flow passage to scavenge, and moisture remaining in these gas flow passages is discharged.
In the conventional scavenging method, the oxidant gas as the scavenging gas is simultaneously circulated through both the fuel gas flow path and the oxidant gas flow path in the fuel cell when the fuel cell is stopped (see, for example, Patent Document 1). .
JP 2001-351666 A

However, in the conventional scavenging method, in order to remove the water present in the solid polymer electrolyte membrane, a large amount of scavenging gas must be circulated through the fuel gas flow passage and the oxidant gas flow passage at the same time. Requires a large compressor, which leads to an increase in system size and cost.
On the other hand, if the scavenging gas is circulated through the fuel gas flow path and the oxidant gas flow path simultaneously without increasing the capacity of the compressor, the flow rate of the scavenging gas flowing through each gas flow path cannot be increased sufficiently. In order to remove the water present in the inside, the scavenging time must be lengthened, resulting in an increase in power consumption.
Accordingly, the present invention provides a scavenging method for a fuel cell that can efficiently and quickly remove moisture in the fuel cell.

The scavenging method for a fuel cell according to the present invention employs the following means in order to solve the above problems.
The invention according to claim 1 oxidizes the fuel gas flow passage (for example, the fuel gas flow passage 5 in the embodiment described later) of the fuel cell when the power generation of the fuel cell (for example, the fuel cell 1 in the embodiment described later) is stopped. A scavenging method for a fuel cell in which scavenging gas is circulated through an agent gas flow passage (for example, an oxidant gas flow passage 6 in an embodiment to be described later) to scavenge these gas flow passages, the fuel gas flow passage and the oxidation gas A scavenging gas having a pressure higher than the pressure of the gas supplied before stopping the power generation of the fuel cell is passed through any one of the agent gas flow passages to scavenge the one gas flow passage. A scavenging method for a fuel cell, wherein after scavenging is performed, a second scavenging is performed in which a scavenging gas is circulated through the other gas flow passage to scavenge the other gas flow passage.

Moisture in the solid polymer electrolyte membrane of a fuel cell moves from a higher water vapor partial pressure to a lower water vapor based on the water vapor partial pressure difference between the anode and cathode. ing.
In the invention according to claim 1, by performing the first scavenging, it is possible to scavenge one of the gas flow passages and increase the water vapor partial pressure in the one gas flow passage. As a result, the first scavenging can increase the water vapor partial pressure difference between the anode and the cathode of the solid polymer electrolyte membrane, and the moisture in the solid polymer electrolyte membrane moves to the other gas flow path side. Can be promoted. Thereafter, by performing the second scavenging, the other gas flow passage is scavenged, and moisture in the solid polymer electrolyte membrane that has been moved to the other gas flow passage by the first scavenging is removed. It can be led out to the other gas flow passage and discharged. Therefore, the water in the fuel gas flow path, the oxidant gas flow path, and the solid polymer electrolyte membrane can be discharged efficiently and quickly.

According to a second aspect of the present invention, in the first aspect of the present invention, in the first scavenging, scavenging gas having a temperature higher than the internal temperature of the fuel cell before stopping the fuel cell is supplied to the one gas flow passage. It is characterized by supplying.
With this configuration, since the water vapor partial pressure in the one gas flow passage can be further increased, the water vapor partial pressure difference between the anode and the cathode of the solid polymer electrolyte membrane can be further increased. Further, the movement of moisture in the solid polymer electrolyte membrane can be further promoted.

According to the first aspect of the present invention, since the movement of moisture in the solid polymer electrolyte membrane can be promoted, the moisture in the fuel cell can be efficiently and quickly removed without particularly increasing the flow rate of the scavenging gas. The time required for the scavenging process can be shortened.
According to the invention which concerns on Claim 2, the movement of the water | moisture content in a solid polymer electrolyte membrane can further be accelerated | stimulated, and drainage property further improves.

Embodiments of a scavenging method for a fuel cell according to the present invention will be described below with reference to the drawings of FIGS.
FIG. 1 is a schematic configuration diagram showing an example of a fuel cell system capable of implementing a fuel cell scavenging method according to the present invention. In this embodiment, the fuel cell system is mounted on a fuel cell vehicle.

  The fuel cell 1 is of a type that obtains electric power by chemically reacting a reaction gas. For example, the fuel cell 1 is formed by sandwiching a solid polymer electrolyte membrane 2 made of a solid polymer ion exchange membrane or the like between an anode 3 and a cathode 4 from both sides. A plurality of cells are stacked (only a single cell is shown in FIG. 1), hydrogen gas is supplied as fuel gas to the fuel gas flow path 5 on the anode 3 side, and the oxidant gas flow path 6 on the cathode 4 side. When oxygen-containing air is supplied to the oxidant gas, hydrogen ions generated by the catalytic reaction at the anode 3 pass through the solid polymer electrolyte membrane 2 to the cathode 4 and undergo an electrochemical reaction with oxygen at the cathode 4. To generate electricity and produce water. Since part of the generated water generated on the cathode side permeates the solid polymer electrolyte membrane 2 and back diffuses to the anode side, the generated water also exists on the anode side.

  The air is pressurized to a predetermined pressure by a compressor 7 such as a supercharger and supplied to the oxidant gas flow passage 6 of the fuel cell 1 through the air supply passage 8. The air supply flow path 8 includes a cooling device 11, a bypass passage 12 that bypasses the cooling device 11 and circulates air, and a three-way valve type flow switching valve 13. The flow switching valve 13 can switch whether the cooling device 11 is circulated or the bypass passage 12 is circulated. After the air supplied to the fuel cell 1 is used for power generation, it is discharged from the fuel cell 1 together with the generated water on the cathode side to the air discharge passage 9 and is discharged to the exhaust treatment device (not shown) via the pressure control valve 10. The

  On the other hand, the hydrogen gas supplied from the hydrogen tank 15 flows through the hydrogen gas supply flow path 17 and the shutoff valve 20, is reduced to a predetermined pressure by the regulator 16, passes through the ejector 19, and enters the fuel gas flow path 5 of the fuel cell 1. Supplied. The unreacted hydrogen gas that has not been consumed is discharged from the fuel cell 1 as an anode off-gas, sucked into the ejector 19 through the anode off-gas flow path 18, and merged with fresh hydrogen gas supplied from the hydrogen tank 15. Then, it is supplied again to the fuel gas flow passage 5 of the fuel cell 1. That is, the anode off gas discharged from the fuel cell 1 circulates in the fuel cell 1 through the anode off gas passage 18 and the hydrogen gas supply passage 17 downstream of the ejector 19.

An anode off-gas discharge channel 22 having a discharge valve 21 branches off from the anode off-gas channel 18. The exhaust valve 21 is opened as necessary to discharge the anode off-gas when the concentration of impurities (water, nitrogen, etc.) in the hydrogen gas circulating through the fuel cell 1 becomes high. The anode off gas discharged from the discharge valve 21 is discharged to the exhaust treatment device, and the anode off gas is diluted with the cathode off gas in the exhaust treatment device.
The air supply passage 8 downstream of the cooling device 11 and the bypass passage 12 and the hydrogen gas supply passage 17 downstream of the ejector 19 are connected by a scavenging passage 24 having a first scavenging valve 23.
The air discharge passage 9 upstream of the pressure control valve 10 and the anode off-gas discharge passage 22 upstream of the discharge valve 21 are connected by a scavenging passage 26 provided with a second scavenging valve 25.

In this fuel cell system, when power generation is stopped, scavenging gas (air) is circulated through the fuel gas flow passage 5 and the oxidant gas flow passage 6 in the fuel cell 1 to be scavenged, and remains in these gas flow passages 5 and 6. Drains moisture to prevent power generation performance from being degraded due to freezing in low-temperature environments.
In particular, in the scavenging process in this fuel cell system, first, air having a higher temperature and pressure than the air supplied to the fuel cell 1 as the oxidant gas before power generation stop is circulated through the oxidant gas flow passage 6 as the scavenging gas. Then, scavenging of the oxidant gas flow passage 6 is performed (hereinafter referred to as first scavenging), and then air as scavenging gas is circulated through the fuel gas flow passage 5 to scavenge the fuel gas flow passage 5 (hereinafter referred to as “scavenging gas”). , Second scavenging).

FIG. 2 is a schematic diagram for explaining the principle of scavenging. FIG. 2 (A) shows the moisture distribution when power generation is stopped (before scavenging), and moisture is present in the fuel gas flow passage 5, the oxidant gas flow passage 6, and the solid polymer electrolyte membrane 2, and is particularly solid. Water is dispersed in the polymer electrolyte membrane 2.
FIG. 2B shows the initial stage of the first scavenging, and FIG. 2C shows the final stage of the first scavenging. The scavenging gas (air) of high temperature and high pressure is passed through the oxidant gas flow passage 6. The water in the oxidant gas flow passage 6 is discharged out of the system and the water present in the solid polymer electrolyte membrane 2 is moved to the anode 3 side.
FIG. 2D shows the time of the second scavenging, and the moisture that has moved to the anode 3 side of the solid polymer electrolyte membrane 2 is led out to the fuel gas flow passage 5 and together with the moisture in the fuel gas flow passage 5. It is discharged out of the system.

The principle that the moisture present in the solid polymer electrolyte membrane 2 moves to the anode 3 side by the first scavenging will be described in detail.
The water in the solid polymer electrolyte membrane 2 of the fuel cell 1 moves from the higher water vapor partial pressure to the lower water vapor based on the water vapor partial pressure difference between the anode 3 and the cathode 4, and the water vapor moves more easily as the water vapor partial pressure difference increases. It has properties.
Since water vapor is a gas, the partial pressure ratio of the water vapor partial pressure to the gas phase total pressure is kept constant when compression or expansion is performed (water vapor partial pressure / total pressure = constant). That is, in the region below the saturated vapor pressure, if the total pressure is doubled, the water vapor partial pressure is also doubled.

Now, it is assumed that air containing water vapor having a water vapor partial pressure W (kPa) under atmospheric pressure P (kPa) is supplied to the gas passage on one pole of the fuel cell (in this embodiment, the oxidant gas flow passage 6). If the water vapor partial pressure in the gas path of the other electrode (in this embodiment, the fuel gas flow path 5) is Y (kPa), the pressure of the air supplied to the one gas path is equal to the atmospheric pressure P (kPa). In this case, the vapor pressure difference ΔP (kPa) between the two electrodes (the anode 3 and the cathode 4) of the solid polymer electrolyte membrane 2 is ΔP = W−Y.
On the other hand, when the pressure of the air supplied to the one gas passage is increased to twice the atmospheric pressure P (kPa), the vapor pressure difference ΔP between the two gas passages due to the constant partial pressure ratio. Becomes ΔP = 2W−Y.
When 2W exceeds the saturated vapor pressure Z (kPa), ΔP = Z−Y. However, since the saturated vapor pressure Z is equal to or higher than the atmospheric pressure W (Z ≧ W), A larger vapor pressure difference ΔP can be generated than when the pressure is equal to the atmospheric pressure P (kPa).

Therefore, by supplying air (scavenging gas) at a pressure higher than the air pressure (oxidant gas pressure) before power generation is stopped in the first scavenging, the vapor pressure difference between the two electrodes can be further increased. The movement of moisture in the molecular electrolyte membrane 2 can be promoted.
Furthermore, since the saturated vapor pressure can be increased by raising the temperature of the supplied air in addition to this, a larger vapor pressure difference ΔP can be generated, and the moisture movement in the solid polymer electrolyte membrane 2 can be reduced. It can be further promoted.
As a result, water in the fuel gas flow passage 5, the oxidant gas flow passage 6, and the solid polymer electrolyte membrane 2 can be efficiently and quickly discharged without specially increasing the flow rate of air as the scavenging gas. And the time required for the scavenging process can be shortened.
And since the water | moisture content in the fuel cell 1 can fully be removed at the time of an electric power generation stop, favorable electric power generation performance can be maintained even in a low temperature environment.

  The scavenging process starts with the ECU 30 starting from the input of an OFF signal of the ignition switch (IGSW) 31 to an electronic control unit (hereinafter abbreviated as ECU) 30, the compressor 7, the flow path switching valve 13, and the pressure control. This is performed by controlling the valve 10, the discharge valve 21, the first scavenging valve 23, and the second scavenging valve 25.

The scavenging process in this embodiment will be described with reference to the flowchart of FIG.
The flowchart shown in FIG. 3 shows a scavenging process routine, and this scavenging process routine is executed at regular intervals by the ECU 30. First, in step S101, whether or not the ignition switch is turned off, in other words, power generation is stopped. It is determined whether or not. If the determination result in step S101 is “NO” (IGSW: ON), the execution of this routine is terminated.
When the determination result in step S101 is “YES” (IGSW: OFF), the process proceeds to step S102, and first scavenging of the oxidant gas flow path 6, that is, first scavenging is performed.
In the first scavenging, the ECU 30 operates the compressor 7, opens the pressure control valve 10, closes the first and second scavenging valves 23, 25, and further air bypasses the cooling device 11 and flows through the bypass passage 12. Thus, the flow path switching valve 13 is switched. That is, the air pressurized by the compressor 7 flows around the cooling device 11 to make the temperature of the air to be supplied to the oxidant gas flow path 6 higher than the internal temperature of the fuel cell 1. By controlling the control valve 10 to a predetermined level, air having a pressure higher than that during power generation is caused to flow through the oxidant gas flow passage 6.

Next, it progresses to step S103 and it is determined whether predetermined time passed. If the determination result in step S103 is “NO” (predetermined time has not elapsed), the process returns to step S102, and scavenging of the oxidant gas flow passage 6 is continued.
If the determination result in step S103 is “YES” (predetermined time has elapsed), it is determined that scavenging for the oxidant gas flow passage 6 has been completed, and the process proceeds to step S104, where scavenging for the fuel gas flow passage 5, ie, Perform scavenging.
In the second scavenging, the ECU 30 continues the operation of the compressor 7 and controls the pressure control valve 10, the discharge valve 21, and the first and second scavenging valves 23 and 25 to open. In the second scavenging, the temperature of the scavenging gas is not limited, and the control for the flow path switching valve 13 in step S104 may be performed by switching the flow path switching valve 13 so that air flows to the cooling device 11. Alternatively, the flow path switching valve 13 may be switched and controlled so that air flows through the bypass passage 12.
Thereby, in the second scavenging in this embodiment, a part of the air pressurized by the compressor 7 flows through the fuel gas flow passage 5 and scavenges the fuel gas flow passage 5, and the remaining air flows through the oxidant gas. Circulates the road 6.

Next, it progresses to step S105 and it is determined whether predetermined time passed. When the determination result in step S105 is “NO” (predetermined time has not elapsed), the process returns to step S104 and the scavenging of the fuel gas flow passage 5 is continued.
If the determination result in step S105 is “YES” (predetermined time has elapsed), it is determined that scavenging of the fuel gas flow passage 5 has been completed, the process proceeds to step S106, and the first and second scavenging valves 23, 25 are closed. Then, the scavenging is finished by stopping the compressor 7, and the execution of this routine is finished.

  Note that the determination process in step S103 may be based on the differential pressure between the upstream and downstream of the oxidant gas flow passage 6 instead of the determination based on the passage of time. This is because the differential pressure decreases as the amount of water in the oxidant gas flow passage 6 decreases, so that the completion of scavenging of the oxidant gas flow passage 6 can be determined by setting a predetermined differential pressure as a threshold value. The same applies to the determination process in step S105. Instead of the determination based on the passage of time, the determination may be based on the differential pressure between the upstream and downstream of the fuel gas flow passage 5.

[Other Examples]
The present invention is not limited to the embodiment described above.
For example, in the above-described embodiment, scavenging gas (air) is circulated through the oxidant gas flow passage 6 in the first scavenging, and scavenging gas is circulated through the fuel gas flow passage 5 in the second scavenging. Conversely, the scavenging gas may be circulated in the fuel gas flow passage 5 in the first scavenging, and the scavenging gas may be circulated in the oxidant gas flow passage 6 in the second scavenging.

It is a schematic block diagram which shows an example of the fuel cell system which can implement the scavenging method of the fuel cell which concerns on this invention. It is a mimetic diagram explaining the principle of scavenging concerning this invention. It is a flowchart in one Example of a scavenging process.

Explanation of symbols

1 Fuel Cell 5 Fuel Gas Flow Path 6 Oxidant Gas Flow Path

Claims (2)

A scavenging method for a fuel cell in which scavenging gas is circulated through the fuel gas flow passage and the oxidant gas flow passage of the fuel cell when power generation of the fuel cell is stopped.
One of the fuel gas flow passage and the oxidant gas flow passage is circulated through a scavenging gas whose pressure is higher than the pressure of the gas supplied before stopping the power generation of the fuel cell. A scavenging of a fuel cell, wherein after the first scavenging is performed to scavenge the flow passage, the second scavenging is performed by flowing the scavenging gas through the other gas flow passage and scavenging the other gas flow passage. Method.   2. The scavenging of the fuel cell according to claim 1, wherein in the first scavenging, a scavenging gas having a temperature higher than an internal temperature of the fuel cell before the fuel cell is stopped is supplied to the one gas flow passage. Method.

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